From patent EP1612403, it is known a fuel injector for an internal combustion engine comprising: a casing having a nozzle at one end for injecting filet into a cylinder of the engine, a movable needle for opening and closing the nozzle, a rod housed in the casing and sliding along its own axis to control movement of the needle, and a metering servovalve housed in the casing.
The metering servovalve comprises a control chamber, which communicates with a fuel inlet and with an outlet channel having a calibrated portion. The pressure in the control chamber controls the axial sliding of the rod, for the purpose of opening and closing the nozzle, and is adjusted by controlling an actuator comprising an electromagnet and a spring.
The actuator operates the translation of a sleeve between a closed position and an open position of the outlet channel. The sleeve is mounted so that it can slide in a substantially fluid-tight manner on an axial stem, which forms part of a fixed valve body with respect to the casing. The outer lateral surface of the axial stem defines an annular chamber into which the outlet channel exits. In the closed position, the sleeve closes the annular chamber in such a way as to be subjected to an axial fuel-pressure resultant that, at least in theory, is null.
In this system, where the metering servovalve and its sleeve are of the so-called “balanced” type, the preloading forces demanded of the actuator spring and the overall dimensions are reduced. In particular, even with small sleeve lifts, it is possible to obtain large fuel passage sections, with consequent advantages in the injector's dynamic behaviour, to reduce sleeve rebound phenomena at the end of opening and closing travel.
The inner diameter of the sleeve is greater than the outer diameter of the axial stem by an amount equal to a diameter clearance, which is preferably less than approximately 5 micron to ensure fluid tightness even without the use of proper gaskets.
It has been noted that the fluid seal between the sleeve and the valve body might not take place in correspondence to the inner diameter of the sleeve, but effectively in correspondence to a mean seal diameter that is larger, due to two phenomena: (1) in use, the sleeve tends to deform under the pressure; and (2) sealing does not take place along a circumference defined by a sharp edge (or null-radius bevel).
With regards to the first phenomenon, it is evident that the pressure of the fuel in the annular chamber reaches relatively high levels, around 1600-1800 bar for example, when the sleeve is in the closed position, while in the discharge area, or rather downstream of the sealing zone, pressure levels are relatively low, around a few bar. Therefore, the pressure in the annular chamber generates a radial force on the sleeve that is outwardly directed and that deforms the sleeve.
This deformation has the effect of “widening” the end of the sleeve and consequently increasing the diameter where contact and sealing on the valve body takes place, with respect to the inner diameter of the sleeve in the non-deformed state.
With regards to the second phenomenon, due to technological/constructional reasons, in practice the contact zone between the sleeve and the valve body is not exactly defined by a circumference, but by an annulus, even if of relatively small radial width. Sealing does not take place in correspondence to the inner diameter of this annulus, but in correspondence to a mean diameter, which is obviously greater than the inner diameter of the sleeve.
The increase in the diameter where sealing takes place with respect to the inner diameter of sleeve in the non-deformed state has the effect of creating an axial unbalancing force. which acts on the sleeve in the direction corresponding to its opening.
The magnitude of the axial unbalancing force depends on the fuel supply pressure and the annulus-shaped area defined by the difference between the diameter in which sealing effectively takes place and the minimum inner diameter of the sleeve at the opposite end.
In order to compensate for the axial balancing force, the actuator spring must have a greater preload force with respect to that theoretically determined by design with a perfectly balanced sleeve, from the axial pressure standpoint, to keep the sleeve closed.
On one hand, the spring's larger preload forces result in larger accelerations and aster impact speeds on closure against the valve body and, in consequence, greater risks of wear and damage to the metering servovalve.
On the other hand, the spring's larger preload forces result in greater risk of so-called “adhesive” wear between the surfaces of the sleeve and the valve body when they come into contact.
To limit the preload of the spring, known solutions have certain constructional expedients to eliminate the axial unbalancing force.
In particular, the sleeve and the valve body are made using materials with high hardness levels. In addition, the geometry and the material chosen for the end of the sleeve are such as to provide the sleeve with high rigidity, so as to practically eliminate elastic deformations.
Nevertheless, the geometry chosen to increase the rigidity results in an increase of the mass of the sleeve and therefore the amount of contact momentum with the valve body during closure. In consequence, the sleeve is subjected to undesired rebounding against the valve body during closure.
Due to these rebounds, on one hand, the metering servovalve does not close immediately, resulting in a greater quantity of fuel being injected into the cylinder than that determined by design.
On the other hand, in spite of choosing materials with high hardness levels, the rebounds cause relatively rapid wear on the circular edge of the sleeve that makes contact with the valve body during closure. This wear results in a progressive increase in the mean diameter at which the seal is created. and therefore an increase in the axial unbalancing force.
As the axial unbalancing force progressively increases, the behaviour of the metering servovalve and the injector as a whole progressively changes over time with respect to that determined by design: the change cannot. be predicted and therefore it cannot be compensated for in any way.
The consequences of this phenomenon are a rapid and significant increase in the flow of fuel recycled to the discharge and a shorter life for the injector.